1. Field of the Invention
The present invention relates to improvements in valve gate piston assemblies typically used in hot runner plastic injection molds. In particular, the present invention relates to improved air piston cylinder apparatus and method that provide improved sealing, improved heat-management properties, less expensive parts, and longer seal life.
2. Related Art
Hot runner valve gate assemblies are well known in the art of injection molding. U.S. Pat. No. 4,173,448 to Rees shows a typical arrangement in which the valve stem is moved up and down by an air piston housed in a cylinder. A valve bushing is mounted in the hot runner manifold and is designed to seal the plastic within the melt channel. However, one drawback of this design is that the cylinder walls are required to transmit all the considerable compressive forces passing from the manifold to the mold backing plate. At the same time, the cylinder walls must effect a proper air seal between the air supply channels in the backing plate and the cylinder contained in the bushing, for pneumatic operation of the double-acting piston. Moreover, the backing plate bore depth must be manufactured to a very tight tolerance in order to achieve the fine balance of sufficient compression to prevent air or resin leakage while not overstressing the components of the assembly. The various components of the valve gate assembly are likewise tightly toleranced, all of which add greatly to cost.
In addition, current cylinder walls must be strong enough to resist the side forces acting on them as the manifold thermally expands laterally thereby causing the bushing assembly to slide across the face of the mold backing plate. In the ""448 Patent design, the cylinder walls also contain conduits for the air to be carried from the backing plate to the cylinder space underneath the piston so that a double acting motion of the air piston can be effected. For this latter reason the cylinder walls are thickened locally. This presents another problem with current cylinder design in that heat from the manifold conducted along the cylinder walls causes the interior surface of the cylinder to be at an elevated temperature thereby drastically shortening the life of the piston seal that must slide along the surface of that wall. Furthermore, heat conducted to the piston via the valve stem (which is directly surrounded by the hot plastic melt stream) also causes the piston to be at an elevated temperature, further contributing to the deterioration of the piston seal. Also, to change the seals, the backing plate must be removed so that the piston can be extracted from the back of the cylinder. Another valve gate piston assembly is shown in U.S. Pat. No. 3,037,245 to Darnell, which shows the air piston housed in a cylinder mounted entirely in the backing plate. The valve bushing 28 is a separate piece mounted in the manifold in FIG. 1, or as an attachment to the cylinder, as shown in FIG. 2. These components are very expensive due to the tight tolerances required for structural integrity and adequate sealing forces. In addition, the thick walls of the cylinder and the piston itself again cause premature deterioration of the piston seal by overheating. The compressive sealing force is transmitted from the manifold solely via the bushing walls to the back plate.
U.S. Pat. No. 4,213,751 to Fernandez, relies on the nozzle housing to seal the plastic at 44 with the valve stem. The air piston assembly is located entirely in the backing plate so that a low temperature environment is available for good seal life. All the compressive forces are transmitted via a separate bushing 70 directly from the manifold to the backplate, none of the force is transmitted through the cylinder wall. However, because the air piston assembly is fixed in the backplate, unable to slide laterally with the manifold as it thermally expands, the valve stem 42 must bend to accommodate this relative movement between the two. Consequently, the stem tends to wear the sealing diameter inside the nozzle housing at 46, eventually causing plastic to leak at that point Furthermore the bending of the stem will cause the piston bushing 68 to wear allowing air from the cylinder to leak. Similarly, U.S. Pat. No. 4,469,191 to Gellert exhibits the same characteristics of a bending valve stem. The ""191 Patent provides no teaching regarding how the compressive forces from the manifold are transmitted to the backplate.
U.S. Pat. No. 5,022,846 to Schmidt shows another bending stem configuration. However, in the ""846 Patent, the valve bushing sealing the plastic is mounted in the manifold and acts as the bottom seal for the air in the piston assembly. A graphite piston seal is used instead of the less costly, but more temperature sensitive, polymer seals. The ""846 Patent seal runs directly inside a through-hole bore within the backing plate, which does not require tight bore depth tolerances but which may require expensive surface finishing of the bore. Also, the bending action of the stem causes wearing of the bushing in the manifold. The compressive forces are transmitted entirely through the bushing 56 from the manifold to the backplate.
U.S. Pat. No. 5,071,340 to LaBianca shows a conventional valve bushing arrangement with a sliding seal in the plate. However, the air supply to the piston is routed through the valve stem to provide cooling thereof. There is no reduction in the temperature of the surfaces on which the air piston seals operated, and all the compressive forces from the manifold are transmitted via the cylinder wall to the backplate.
U.S. Pat. No. 5,374,182 to Gessner shows another embodiment of a conventional valve bushing with sliding seal. In the ""340 Patent, the bushing 130 and cylinder body 136 are two pieces fastened together with a nut 158. Tight tolerances are still required for the cylinder and the plate bore depth. Additionally, the cylinder walls and piston, design do nothing to lower the surface temperature where the piston seals operate. All the compressive forces from the manifold are transmitted via the cylinder wall to the backplate. The ""340 Patent, like all the other configurations in which the thermal compression forces are used to maintain the air seal between the cylinder and the backplate, requires that the manufacturing tolerances of the components making up the stack be very closely controlled, typically xc2x10.0005xe2x80x3. Obviously, this makes manufacturing very costly. For example, the bore depth in the backplate typically has a tolerance of +0.0002xe2x80x3. Furthermore, the compression forces transmitted through the cylinder walls are on the order of 10,000-20,000 lbs, which requires the components transmitting these forces to be made of steel or equivalent high strength materials that even in comparatively thin sections are still good conductors of heat.
U.S. Pat. No. 5,478,230 to McGrevy discloses a valve gate assembly mounted in a back-to-back configuration suitable for use in a stack mold. In this arrangement, the valve piston assemblies are mounted in a common cylinder housing inside the manifold. Air supply to both sides of the piston is routed along the outside of the cylinder walls. However, there is no disclosure of using the air to cool the surface along which the piston seals operate. Indeed ""230 Patent discloses that the seals are made from expensive xe2x80x9creinforced high temperature plasticxe2x80x9d(col. 5 ln 12). To change the seals, the support plates and manifold assembly must be dismantledxe2x80x94a time consuming and costly operation.
There is, therefore, a need for a valve gate air piston assembly in which manufacturing tolerances can be increased, low cost piston seals can be used, effective operation over a long service life can be achieved, and convenient access to the seals can be provided for easy serviceability.
The present invention provides an improved valve gate piston assembly for use in hot runner molds in which only part of the compressive forces between the manifold and the backplate are transmitted through the piston cylinder, to maintain the air seal against the mold plate. Preferably, the piston cylinder uses an integral spring or resilient means to provide sufficient air sealing forces, thus allowing the constituent components of the assembly to be made with less precision and less cost. Preferably, a backup pad is used as a second path to transmit the compressive forces between the manifold and the backplate. This second path also transfers a good amount of heat from the manifold to the backplate, thus lowering the temperature of the piston cylinder, leading to a longer piston seal life.
According to a first aspect of the present invention, air cylinder apparatus for a hot runner nozzle assembly includes an air piston cylinder dimensioned to form an air seal with an air piston, and disposed to transmit less than all of the compressive force between the hot runner manifold and the backplate. A spring is disposed to flex when the compressive force is applied between the hot runner manifold and the backplate.
According to another aspect of the present invention, apparatus for an air piston of a hot runner nozzle assembly includes an air piston cylinder for engaging the air piston The air piston cylinder provides a first path for transmitting the compressive force between the hot runner manifold and the backplate. A backup pad is disposed to provide a second path to transmit the compressive force between the hot runner manifold and the backplate.
According to a further aspect of the present invention, an air cylinder for a hot runner nozzle includes a piston cylinder in air communication with an air channel in the backplate. The piston cylinder is disposed to transmit less than all of the compressive force between the manifold and the backplate to cause an air seal to be maintained between the piston cylinder and the air channel.
According to yet another aspect of the present invention, apparatus for an air piston in a hot runner assembly includes a piston cylinder forming an air seal with the air piston, and in air communication with an air channel in the hot runner backplate. The piston cylinder transmits less than all of the compressive force and less than all of the heat transfer between the backplate and a manifold. A backup pad is disposed in contact with the manifold and the backplate, for transmitting both the compressive force and the heat transfer between the backplate and the manifold. A flex member is provided for absorbing compressive force between the backplate and the manifold to maintain the air seal between the piston cylinder and the air channel.
According to still another aspect of the present invention, a method of sealing an air piston in a hot runner nozzle assembly, includes the steps of: (i) sealing the air piston with a piston cylinder disposed between the hot runner manifold and the backplate; and (ii) causing compressive forces to be transmitted between the hot runner manifold and the backplate via the piston cylinder and the via a backup pad.